Stark et al. showed that the synthetase responsible for making ppaA2'pA2'pA (2-5A) from ATP is present in many different cells and that the level of this enzyme increases upon arrest of growth (mammalian cells) or withdrawal of hormones (chick cells). Since 2-5A causes mRNA degradation in virus-infected, interferon-treated cells of these species, a more general role for the 2-5A system in regulated mRNA degradation was suspected but could not be tested because the synthetase does not function in the absence of an activator (dsRNA in the case of infected cells). A direct radioimmunoassay for 2',5'-linked oligoadenylates has been developed which is specific for the moiety -pA2'pA2'pA-\at nanomolar levels. By use of this assay, 2-5A has now been found to be present at 10 to 30 nM in chick oviducts 1 to 3 hours after withdrawing estrogen, in rat mammary glands 4 days after removing pups and in human neuroblastoma cells in culture 1 to 2 days after arrest of growth at confluence. In addition to 2-5A itself, other 2',5'-linked oligomers have been found in all of these systems by HPLC chromatography. In monkey CV1 cells infected with SV40 and then treated with interferon, these compounds can reach levels of up to 1 micromolar, although 2-5A itself can barely be detected. The nature and significance of these unusual oligomers will be investigated, as will be the function and control of the 2-5A system in the absence of interferon treatment and virus infection.